1. Field of the Invention
This invention relates to foam-coated rolls that can be used in electrostatic copiers and various types of printers as hot rolls and as pressure rolls in fixing assemblies, as cleaning rolls in cleaning devices, and as transport rolls for recording media such as paper. This invention also relates to a method for preparing these rolls.
2. Background Information
The increasingly widespread use of electrophotographic copiers and laser printers has been accompanied by efforts to reduce the size and weight of these devices. The toner-fixing assembly used in these devices has accordingly also been reduced in scale, and a lower hardness has been required of the rubber-coated rolls used therein. In particular, a reliable nip width is required to achieve a stable paper transport between the pressure roll and hot roll.
Pressure rolls whose surface is covered with a single layer of silicone rubber have not exhibited the following property combination: low hardness plus a low coefficient of friction plus a smooth, flat surface plus wear resistance plus excellent paper-transfer performance. This has resulted in the use of multi-layer elastic rolls comprising a foam layer formed around a core member and a top layer of a fluororesin having a low coefficient of friction and excellent toner-release performance.
With regard to methods for fabricating multi-layer pressure rolls, Laid Open Japanese Patent Application [Kokai or Unexamined] Number Sho 59-61864 [61,864/1984] discloses a method whereby a heat-curable silicone rubber foam composition is applied over a metal roll core and subsequently foamed and cured to yield a foam roll. After the surface of this foam roll has been polished, a fluororesin latex is applied to the foam layer and then baked.
Laid Open Japanese Patent Application [Kokai or Unexamined] Number Hei 2-253285 [253,285/1990] teaches a method wherein an ambient temperature-curing rubber is used to bond the polished foam roll to a fluororesin film tube functioning as the outermost layer.
Unfortunately, both of these methods employ a complicated fabrication process with the associated negative cost consequences.
Other types of methods are disclosed in Japanese Patent Publication Number Hei 6-2353 [2,353/1994] and in Comparative Example 4 of Laid Open Japanese Patent Application [Kokai or Unexamined] Number Hei 2-253285 [253,285/1990]. In these methods, fluororesin tube-covered elastic rolls are fabricated by placing a fluororesin tube over the circumference of a metal roll core, injecting a heat-curing silicone rubber foam composition containing an organic compound as the blowing agent into the gap between the metal roll core and fluororesin tube, and then foaming and curing the curing the foam composition.
All of these prior art methods, however, require the application of high temperatures, typically from 150.degree. C. to 220.degree. C., to achieve foaming and curing. Because the heating period causes such phenomena as thermal expansion and thermal shrinkage to occur in the fluororesin tube and foam layer, there is a strong tendency in these methods for surface creasing to occur, and the fabrication of flat, smooth elastomer-coated rolls is thereby rendered highly problematic.
One objective of the present invention is to provide a foam-coated roll exhibiting a high level of toner release and the absence of problems associated with prior art rolls. A second objective of the present invention is to provide a highly productive method for fabricating this type of roll.